Satellite navigation systems operate in multiple frequency bands in order to reduce multipath effects and ionospheric or tropospheric errors so as to ultimately provide enhanced positioning accuracy to the user. The existing GPS (Global Positioning System), for instance, uses signals in the L1 frequency band, centred at 1575.42 MHz, and in the L2 band, centred at 1227.6 MHz. The coming European Galileo positioning system will operate in a different set of frequency bands, e.g. the E5 band (1164-1215 MHz), the E6 band (1260-1300 MHz) and the E2-L1-E1 band (1559-1593), called hereinafter “L1-band” for simplicity. In order to profit from the increased positioning capabilities and to be able to use different positioning services, a user needs receiver/transmitter infrastructure capable of operating at a plurality of frequencies.
Multi-band stacked patch antennas are known in the field of satellite positioning systems. A multi-frequency antenna with reduced rear radiation and reception is e.g. disclosed in US patent application 2005/0052321 A1. Such a multi-band antenna typically comprises a stack of dielectric substantially planar substrates, with a conductive layer disposed on a surface of each substrate. Each conductive layer is associated with a specific frequency band and configured so as to be resonant within the respective frequency band. The patches are parasitically coupled through slots to feeding microstrip lines applied on the rear surface of the undermost dielectric substrate. Another antenna for satellite positioning applications is described in “A Dual Band Circularly Polarized Aperture-Coupled Stacked Microstrip Antenna for Global Positioning Satellite”, Pozar et al., IEEE Transactions on Antennas and Propagation, Vol. 45, No. 11, November 1997. Pozar's antenna includes a stacked arrangement of first and second antenna patches, a crossed slot feed and a microstrip feed network. The latter includes power combiners to sum the signals of the microstrips with the correct relative phase.
Other antennas, not specifically related to satellite positioning applications and/or multi-band operation are e.g. known from US 2004/0189527 A1 disclosing a crossed slot-fed microstrip antenna, U.S. Pat. No. 6,054,953 an aperture-coupled dual-band antenna, US 2004/0263392 A1 a multi-band base station antenna for communicating with terrestrial mobile devices and US 2004/0239565 A1 a printed dual-band antenna.
Important issues in satellite positioning systems are multipath effects and phase-centre stability. Multipath signals are due to reflections at surfaces in the surroundings of the antenna and they constitute a limiting factor for the determination of position. The nearer the reflecting surface is to the antenna, the more difficult it becomes for the receiver to mitigate the effect of multipath. In order to reduce short-distance multipath effects, the reception pattern of the antenna has to be tailored.
Phase centre variations over frequency are another limiting factor for position determination and also have to be minimised at antenna level. The change of the phase centre with temperature is a further parameter, which shall be minimised.
In satellite navigation systems, typical signal levels are of the order of −130 dBm (L1 band) and −125 dBm (E5/E6 band), which sets relatively severe requirements for the RF front end. Additionally, out-of-band rejection shall be very high, especially if the antenna is to be used in an environment with high RF interference levels, such as e.g. avionics.
Another important point is group delay variation with frequency. Group delay is mainly due to those parts of electric circuits that are based on resonant sections. Group delay variations shall be kept low over a given frequency band so that the position can be accurately determined. Additionally, change of group delay with temperature for a given frequency shall be minimised.